I'd set up a feeding station for the ravens somewhere on the opposite side of the yard from the chickens, and get them used to a routine of being fed over there. I don't think you'll be able to get them to leave unless they want to leave, I'd just try to train them to stay away from your run.
Killing them is illegal in most places, and if you yell at and chase them, you'll only make an enemy of them, and ravens have a long memory.